Actually I'm being stupid, writing to a folder in c:\ probably is allowed in Vista, I'm not certain though. Even so, the other point still stands.
As far as multiple installation goes the -r switch will presumably still be supported so you could install mIRC normally which would use UserFolder\Application Data\mIRC, manually create UserFolder\Application Data\mIRC2 and UserFolder\Application Data\mIRC3, and then just make extra shortcuts to the original mIRC.exe using the -r switch to use the additional folders for settings.
